There has been a recent upsurge in demand for highly advanced techniques for water purification capable of removing not only physical contaminants such as organic materials, heavy metals, etc. but also biological impurities such as viruses. Such a water purification system typically includes a membrane filter having pores smaller than particles that are to be filtered out. Examples of the membrane filter include a microfiltration filter (MF; pore size 50˜2000 nm), an ultrafiltration filter (UF; pore size 1˜200 nm), and a reverse osmosis filter (RO; pore size 0.1˜2 nm). The membrane-based liquid filter/separation techniques are regarded as very important in water treatment fields including oil/water emulsion separation and desalting, since they are very effective in separating fine particles, bio macromolecules, oil/water emulsions, salts, and ultrafine particles such as viruses. The RO or UF membrane is capable of removing particles larger than 60 nm, and hence is used to remove bacteria or toxic viruses from water, air or blood. The size of pathogenic viruses such as the SARS virus and avian influenza virus is in ranges of 80˜200 nm. However, in order to remove ultrafine particles (virus) of 30 nm or less, the size of pore must be much smaller. This results in a drastic pressure drop and reduces process flow rates. In addition, during use, membranes are susceptible to clogging which further degrades the flow rates, and back washing must be used. Back washing markedly increases operating costs and undesirably causes membrane damage or pore size increase. Accordingly, there has been a demand for a filtering device having a low operating pressure and an improved filtration efficiency in a large scale plant.
Mesh filters or non-woven fabric filters are known to have low pressure drop. A fibrous depth filter is a non-woven fabric filter composed of layers of randomly oriented fibers (LROF). The porous structure is defined by gaps between the fibers, and thus pores become smaller in proportion to an increase in the thickness of the filter layer. When having a proper thickness, the filter can retain fine particles by size exclusion. This filter is capable of filtering 85˜95% by weight of fine particles but cannot filter ultrafine particles such as viruses.
Melt-blown non-woven fabrics usually have a fiber diameter of 1 μm or more and thus a filter made thereof cannot filter nanoparticles such as viruses. Even when ultrafine fibers having a diameter distribution of 5˜500 nm are used, fibers having a larger diameter are present so that large pores are formed, undesirably decreasing the level of filtering precision and making it difficult to remove water-borne viruses having a size of 10˜100 nm.
On the other hand, ultrafine fibers having a diameter corresponding to 1/10˜ 1/1000 of the diameter of melt-blown fibers may be manufactured using electrospinning. Non-woven fabric filters manufactured using this type of fiber have an operating pressure much lower compared to an MF filter using a porous membrane. However, it is very difficult to increase the level of filtering precision enough to remove nanoparticles such as viruses, while maintaining low operating pressures and high flow rates. The reason for this is that there is a limit in decreasing the pore size sufficiently to filter ultrafine particles such as viruses, by minimizing the fiber fineness, and also that a small pore size drastically increases the operating pressure while undesirably sharply decreasing the flow rate.
International Publication No. WO 07/054040 discloses various polymeric nanofiber filters. However, these polymeric nanofiber filters suffer from a short lifespan, low thermal stability, swelling properties in various solvents, and difficulties in surface modification.
In contrast, a ceramic nanofilter mainly used to purify wastewater has higher corrosion resistance and mechanical strength, and a long lifespan. Specifically, whereas the polymer filter is easily damaged during steam cleaning or chemical processes periodically conducted to remove contaminants, the ceramic filter is stable even at a high temperature of 500° C. and is chemically inactive, thereby enabling easier maintenance in terms of washing and regeneration.
The ceramic filter is typically manufactured from a sol-gel solution of a metal oxide precursor, and comprises a support layer provided in the form of a thin film having pores with a size of 1 μm and an uppermost layer having nano-sized pores. The pores of the ceramic filter are formed by voids between ceramic particles, in which the ceramic particles having different sizes are arranged in a layer-by-layer deposition form, thus forming a ceramic membrane having a gradation structure. However, in the sol-gel process, it is often difficult to control the pore size because of the particles having an irregular shape, and undesirable cracks or pinholes may be formed in the uppermost layer during drying and sintering processes. Also, when pore size is decreased to increase selectivity, a serious loss in the permeation flow rate and agglomeration of fine particles in the uppermost layer may occur, and thus it is difficult to maintain high selectivity and high permeation flow rate. Furthermore, a dead end pore structure which does not contribute to filtration is formed, and thus the porosity of the separation layer is very low to the extent of 36% or less. Hence, it is very difficult to actually obtain a porous ceramic filter having both superior selectivity and a sufficiently high permeation flow rate.
U.S. Pat. No. 7,601,262 discloses a water treatment composite filter that uses powdery aluminum hydroxide nanofibers in order to remove nano-sized viruses or particles. This filter is manufactured from an alumina sol bound to glass microfibers having a length of 2˜3 mm. Because the aluminum hydroxide nanofibers are powdery and thus cannot form a filter, glass fibers are used to increase mechanical strength and formability of the filter. In order to increase the precision of filtering, the thickness of the alumina filter is doubled but the permeation flow rate is thereby cut by half. Briefly, increase in the mechanical strength of the filter results in a loss in the permeation flow rate.
International Publication No. WO 08/034190 discloses a filter capable of removing ultrafine particles such as viruses which is composed exclusively of powdery metal oxide nanofibers, without a glass fiber support, manufactured by using a suspension of metal oxide nanofibers having a length larger than a diameter and has a pore size of 1˜100 nm. In this case, however, there is a limit to the length of the metal oxide nanofibers which can form a homogeneous suspension, and a non-uniform suspension makes it difficult to manufacture a homogeneous filter. Furthermore, although the filtration efficiency of ultrafine particles such as viruses is very high because of a pore size of 1˜100 nm, the permeation flow rate undesirably decreases.
As described above, conventional filters known to date are still unsatisfactory in terms of filtration efficiency, permeation flow rate, heat resistance, preparation and so on, the properties being required of an excellent water treatment filter material.